As is known in the art, three-dimensional (3D) endoscopy which provides clinicians with depth information can greatly aid a variety of minimally invasive procedures. Depth resolved imaging with a large, three-dimensional field of view is, however, challenging when utilizing flexible imaging probes such as borescopes, laparoscopes, and endoscopes having relatively small diameters.
Confocal imaging through a fiber-bundle using a high numerical aperture (NA) lens such as that described in Y. S. Sabharwal et al., “Slit-scanning confocal microendoscope for high-resolution in vivo imaging,” Appl. Opt. 38, 7133 (1999) is one solution to this problem. The 3D field of view for these devices, however, may be limited to less than a few millimeters due to the small clear aperture of the objective lens and low f-number required for high-resolution optical sectioning.
Other methods, such as stereo imaging and structured illumination as described in M. Chan et al., “Miniaturized three-dimensional endoscopic imaging system based on active stereovision,” Appl. Opt. 42, 1888 (2003) and D. Karadaglic et al., “Confocal endoscope using structured illumination,” Photonics West 2003, Biomedical Optics, 4964-34, respectively, have also been described. These techniques, however, likely a large number of pieces of hardware for the probe than confocal imaging through a fiber bundle. This additional hardware increased the size, cost, and complexity of such devices.
Spectrally-encoded endoscopy (“SEE”) is a techniques which uses a broadband light source and a diffraction grating to spectrally encode reflectance across a transverse line within a sample. A two-dimensional image of the sample can be formed by slowly scanning this spectrally-encoded line across the sample. This exemplary technique generally uses a single optical fiber, thereby enabling imaging through a flexible probe having a small diameter. When combined with interferometry, the SEE technique has the additional capability of providing three-dimensional images.
Depth-resolved imaging can be achieved by incorporating an SEE probe into the sample arm of a Michelson interferometer. Using this arrangement, two-dimensional (“2D”) speckle patterns can be recorded by a charge-coupled device (“CCD”) camera at multiple longitudinal locations of a reference mirror. Subsequently, depth information can be extracted by comparing the interference obtained at consecutive reference mirror positions.
One problem with this exemplary approach, however, may be that the reference mirror should be maintained in a stationary configuration to within an optical wavelength during a single image (or line) acquisition time to avoid the loss of fringe visibility. Stepping the reference mirror with such high fidelity over multiple discrete depths is quite challenging at the high rates required for real-time volumetric imaging.
One exemplary improvement in an acquisition speed and therefore susceptibility to sample motion can be achieved using time-domain heterodyne interferometry. In this exemplary approach, an interference signal can be recorded with a standard photodetector at every group delay scan of a rapid scanning optical delay line (“RSOD”). By applying a short-time Fourier transformation (“STFT”) to a so-measured trace, transverse and depth information can be extracted. In this exemplary method, three-dimensional (“3D”) data sets can be acquired at a rate of about five per second.
Image quality can be governed, at least in part, by the signal-to-noise ratio (“SNR”). For fast imaging rates and low illumination powers used for safe clinical imaging, maintaining a high SNR may be challenging. For example, with 4 milli-watt (mW) of power on the sample, the time-domain SEE system can provide an SNR of approximately 10 dB.
Using an exemplary SEE technique, a low NA collection system can be utilized to achieve a large working distance, likely resulting in a decrease in the solid angle of collection of light scattered from tissue. Typical optical parameter of NA=0.01 can allow the collection of, e.g., only 0.01% of light scattered from the sample. As a result, the signal of SEE images of tissue may, e.g., only be 10 dB above the noise floor. Increasing the imaging speed increases the bandwidth, thereby decreasing the SNR commensurately.
High imaging speed and high SNR can be important for clinical applications of optical imaging. Optical coherence tomography (“OCT”) is an imaging technique which shares certain principles with 3D SEE. For example, exemplary OCT systems can utilize the RSOD and a single detector, and may be operated with an A-line acquisition rate in the range of about 2-3 kHz. This can correspond to about 4 frames/second with 500 A-lines per frame. An improvement of a number of orders of magnitudes in imaging speed has been demonstrated with an alternative approach, e.g., a spectral-domain OCT (“SD-OCT”) technique. In this exemplary technique, a high-resolution spectrometer can include a diffraction grating and a linear CCD array which may be used to record spectral interference between light from a sample and from a fixed-length reference arm. The improvement in SNR using such exemplary technique has enabled imaging at 30,000 A-lines per second, nearly three orders of magnitude improvement over time-domain methods.
An exemplary proof of principle using spectral domain interferometry for spectral encoding has been described in by Froehly et al., Optics Communications 222 (2003), pp. 127-136. In such document, transverse and depth resolution of this exemplary technique has been analyzed, and a phase-sensitive depth measurement of a 1 mm thick glass plate was demonstrated.
Accordingly, it may be beneficial to address and/or overcome at least some of the deficiencies described herein above.